Diffusion testing of solid samples (e.g. U.S. E.P.A. Method 1315, Mass Transfer Rates of Constituents in Monolithic or Compacted Granular Materials Using a Semi-Dynamic Tank Leaching Procedure, Rev. 0, January 2013 and it predecessors) is traditionally accomplished by immersing a sample in a bath of deionized water, allowing a period of time to pass, collecting the water, replacing it with fresh water, and repeating these steps over numerous specified leaching intervals, typically up to total duration of 63 to 90 days. The collected water is subsequently tested for the presence of inorganic/organic compounds. EPA Method 1315 describes two main configurations for semi-dynamic leach (SDL) testing: 1) radial mass transfer from monolithic and solidified cylinder samples positioned in the center of a water bath (radial SDL test); and, 2) one-dimensional (1-D) mass transfer from the top exposed planar surface of a soil or soil-like media compacted in a cylindrical mold into an overlying water column (1-D SDL test).
Radial SDL tests are typically conducted in large glass jars, for example a 2 liter jar or larger, wherein the sample to be tested is suspended in the center of the jar using a wire or string, or by placing it on a porous mesh pedestal or housing. The jar is then filled with water, then repeatedly exchanged out at the end of each leaching interval. Unidirectional, or 1-D SDL tests may be conducted using a compacted soil or soil-like sample in an open-faced mold that can be placed with a snug fit in the bottom of a cylindrical water bath.
The exchangeable water bath of the SDL test aids in the quantification of leaching rates of various compounds from solids such as soil, sediments, ash materials, cemented media, industrial byproducts, waste media, building materials, and the like, and semi-solid materials such as mine tailings, pastes, gels, tarry wastes, hydrated clays and sludges, drilling muds and the like. However, many sparingly soluble organic compounds may reach their effective saturations in the water bath during a given leaching interval, especially long testing intervals (>2 days). Mass transfer (diffusion or leaching) of these compounds can be greatly diminished or terminated as individual organic compounds approach their effective saturation in the water bath prior to change out.
U.S. Pat. No. 7,930,948 discloses the use of “gel elastomers” coated on the inside wall of jars used for the conventional tests, wherein the gel elastomers are capable of absorbing and retaining the leached compound(s). The positioning of the gel elastomer on the inside surface of the jar facilitates the establishment of a uniform gradient field for diffusion. Polydimethylsiloxane (PDMS) is one example of the gel elastomer. From a fabrication and deployment perspective, the gel elastomer is typically deposited inside the jar as a liquid (uncured) elastomer. The jar is then continuously rotated to create a uniform layer of the elastomer on the inner walls of the jar while curing. With heating, the curing process can take 60 minutes per jar. This fabrication process is cumbersome, time consuming, and an inefficient means to manufacture when a large number of such testing jars.
The gel elastomer is described as providing sufficient capacity to absorb the totality of leaching organic compounds, such that the water bath concentration of each organic compound is maintained at low levels, providing a strong gradient for mass transfer during each leaching interval of the test. In U.S. Pat. No. 7,930,948, the solid phase sample (cylinder/compacted soil) is carefully removed after each leaching interval and placed in a new water bath within a jar having its internal walls coated with PDMS, thus commencing the next leaching interval. The accumulation of organic compounds in both the water bath (eluate) and the gel elastomer (PDMS) liner within the jar used in the previous leaching interval is then measured. The water bath is removed in its entirety for analysis, allowing the PDMS within the jar to be extracted with a fixed amount of solvent to recover the absorbed compounds from the gel elastomer matrix. The solvent is then analyzed for the organic compound(s) of interest.
The gel elastomer coating described in U.S. Pat. No. 7,930,948 is cured on the inside wall of the jar, therefore, it is not removable and limits each jar to a single use. It may only be used for one leaching interval and extraction with one solvent, which limits the analytical work that can be performed with one lined jar. In the case of volatile organic compounds (VOCs), methanol is the preferred extractant for the gel elastomer jar liner. For semi-volatile organic compounds (SVOCs) and pesticides, acetonitrile is the preferred extractant for the gel elastomer jar liner. Accordingly, the simultaneous analysis of different classes of organic compounds from a single gel elastomer jar liner is precluded; multiple solid samples must be tested in parallel (duplicate) if the leaching of both VOCs and SVOCs from the same solid phase sample is required, effectively doubling the materials testing cost.
Subsequent solvent extraction requires the coated jar to be contacted with a known quantity of extractant (solvent), requiring that either the entire jar is filled with solvent, or a lesser quantity be added, which requires rotating the jar to ensure adequate contact of the solvent with all of the gel elastomer. Moreover, multiple solvent extractions of the gel elastomer may be required to remove the entire absorbed mass of organic compounds.
Finally, it is difficult to determine that the amount of gel elastomer in the jar is sufficient for the test; that is, whether sufficient capacity is provided by the amount of thin gel elastomer coating on the jar. Making very thick coatings of gel elastomer on the jar is inefficient and adds to the unit cost of the SDL test.
TestAmerica Laboratories, Inc. of Darien, Conn. made a presentation on Jan. 29, 2014 at the Second Annual RE3 Conference (Philadelphia, Pa.) entitled “Use of a Commercially Available Polyethylene Bag as an Absorptive Medium for Semi-volatile PAH Analysis”, describing a method wherein commercially available zip-top polyethylene bags are inserted into glass jars with the open top portion of the bag protruding from the jar. The solid sample is suspended within the jar and further within the bag, and the bag is filled with water. The bag is employed as the absorptive medium in the radial mass transfer of organic contaminants from the sample. It will be appreciated that this approach solves some of the problems associated with U.S. Pat. No. 7,930,948 discussed above.
However, since the entire polyethylene bag is not immersed in the water, it can be difficult to know the mass of the actual absorptive medium exposed to the water, which in turn makes quantification of leachate concentrations difficult. Additionally, the amount of total polyethylene mass available for absorption of organic compounds may not be sufficient in some cases, since commercially available bags are relatively thin, typically 2 mils (51 μm). Where insufficient mass is provided and saturation of the absorptive medium is reached, radial mass transfer stops until the bag is changed.